In order to explore the role of calretinin in the brain and its regulation at the molecular level, we have cloned and analyzed the rat cDNA coding for calretinin. An immunoreactive clone was isolated from a rat brain cDNA expression library in lambda-gt 11. The 1.45 kb insert was subcloned into the Eco RI site of the pGEM-4Z transcription vector for further analysis. Its identity was confirmed by comparison with human calretinin. The rat cDNA sequence comprised a 54 bp 5'untranslated region, an 816 bp open reading frame (ORF;) including start and stop codons, and a 579 bp 3' untranslated region. The ORF has 271 codons coding for a putative protein of 31.4 kDa. A polyadenylation signal and 13 adenylate residues were found near the 3' end. The evolutionarily conserved calcium binding domains and connecting regions and the limited changes observed between rat and chick primary structure lead us to believe that calretinin interacts with other highly conserved constituents of brain cells. This claretinin cDNA clone provides a new probe for the analysis of specific neurons in the central nervous system. The cDNA probe will allow a more detailed analysis of calretinin expression in the brain and will be useful for screening genomic libraries for the complete chromosomal gene.